The government's plans to sell off up to 30% of the Royal Mail to the private sector have attracted the opprobrium of scores of backbench MPs - but nobody seems to have asked what might happen in such a sell-off to the Postcode Address File (PAF), the list of all the postcodes for 27m addresses in the UK, used by a huge number of organisations for mailshots, geographic targeting, and even by satnav companies for navigation.

Lord Mandelson, the business secretary, will today present a bill in the House of Lords proposing the sale of a minority stake in the Royal Mail, which is seeing a dramatic fall in the volume of letters it handles due to the "technology wedge" of emails and the internet.

PAF and intellectual property are not mentioned in the Hooper report, published in December (http://bit.ly/hooper01), which identified structural weaknesses in the Royal Mail's business model, including its lack of automation.

But PAF is key to the organisation's functions for addressing and routing letters, even though its revenues are small; in August 2007 the postal regulator Postcomm revealed that PAF operations made a profit of £1.58m on revenues of £18.36m, all but £4m from resellers.

One possibility is that a part-private Royal Mail may try to "sweat the assets" - including PAF, which is viewed as essential by many direct marketing organisations - by raising prices.

"The reason for getting the private sector involved [in the Royal Mail] is to improve efficiency," said Robert Keitch, director of media channel development at the Direct Marketing Association. Raising PAF prices would make it harder to check addresses and increase the need for manual checks by postal staff, he suggested.

The Free Our Data campaign has consistently suggested that the PAF - linked to map data - should be made available for free, without copyright restrictions, due to its growing importance for location-based services. The comparatively small cost of running it, especially without the costs of administering its sales and checking for violations of licences, could perhaps be borne through a levy on address or name changes, or simply through the tax revenues that could be gained from new companies set up to take advantage of the datasets. However, it is unknown whether Mandelson will recommend that.

The Royal Mail said: "It's too early to speculate about details relating to the sale of a minority share of Royal Mail."